Hot-melt adhesives are being used in increasingly larger volumes in a number of industrial sectors, including the glazing, automotive, and appliance industries. The growing popularity of the hot-melt adhesives results from the many distinctive advantages that such materials offer over the two main classes of adhesives that they replace, namely, solvent-based adhesives and two-part reactive adhesives. Unlike solvent-based adhesives, hot-melt adhesives contain 100% adhesive solids and are solvent free. When applied to the locus of the adhesive bond, all of the solids are used for their functional purpose, potentially hazardous solvent emissions are eliminated, and no residual solvent is left in the adhesive. Unlike two-part reactive adhesives, which usually require careful mixing of two chemically reactive liquids, hot-melt adhesives are purchased and used as a single, homogeneous product. In addition, the hot-melt adhesives do not exhibit the shrinkage which typically is encountered in the use of two-part reactive adhesives.
At the present time, the advantages achieved by the use of hot-melt adhesives are somewhat offset by the problems of delivering the hot-melt adhesive to the locus of the adhesive bond. One commonly employed dispensing method for delivering hot-melt adhesives to the locus of the adhesive bond involves charging chunks, slugs, or bricks of the solid hot-melt formulation to an extruder in which the formulation is melted and delivered to the locus of the adhesive bond by pumps. Such types of dispensing equipment tend to be cumbersome, costly, and inconvenient to use in that a worker is required to continuously deliver the solid chunks of the hot-melt adhesive to the extruder hopper. A second method that is sometimes employed involves pumping the hot-melt adhesive from a heated liquid reservoir. Disadvantages of such delivery systems have traditionally centered about the high capital costs of such equipment, equipment failures due to the strains developed in pumping the viscous hot-melt adhesive, and an inability to consistently maintain proper operating temperatures in the drum, in the hose, and in the nozzle.
A more recently developed dispensing method for hot-melt adhesives involves equipping a small extruder with auxilliary means for feeding a rod or tape of the solid hot-melt adhesive into the extruder in response to the quantity of melted adhesive delivered from the extruder. Such equipment works well when the rods or tapes are packaged as small reels on a core. When attempts are made, however, to equip such apparatus with larger reels of the rods or tapes, operating problems develop by reason of the tendency of the rods or tapes to block or adhere to each other by reason of the contact pressures developed on the strands in the large rolls. To minimize such blocking problems, suppliers, of the large reels of the rods or tapes of the solid hot-melt adhesives have begun treating the surfaces of such strands with a powder such as mica to eliminate or minimize the blocking problems. Such efforts have been only partially successful.
In view of the difficulties noted above, there is a need in the art for the development of solid hot-melt adhesive formulations in a physical form that can be fed easily to the melting and delivery apparatus over extended periods of time.